Hoard 2.4.2

Fixes from an outside review of 2.3.1. What was found and what changed, finding by finding, is in
docs/security-review-2.3.1-response.md.

• Only Hoard's own window can use Hoard: every request for your library, your downloads, their pictures or
  an action now needs a key that's new each time Hoard starts, even from your own computer. Other programs,
  and other people's accounts on a shared computer, could reach Hoard's address and, for example, point its
  downloads folder somewhere else. Hoard opens its window with a one-time link that the page trades for the
  key. Started as a server from the command line (--no-open, --port or --host), Hoard prints an
  address that includes it.
• Payhip: when Payhip blocks the automated browser, the page listing products for you to download yourself
  only links to Payhip and your own shops, whatever a product's record says (a record could have held a
  javascript: link), and it's written the same careful way as every other file.
• Resumed downloads: a half-finished file is only continued with the part that follows it, and only counts
  as finished when the store says the file ends there. Otherwise it's downloaded again from the start rather
  than stitched together from two different versions, and a file is only put in place once it's as long as
  the store said.
• An emptied downloads folder: catalog.json and tags.json no longer go on listing products that aren't
  there any more (the Unity window showed them).
• Stop: the file that had just finished downloading when you pressed Stop is recorded, instead of being
  downloaded again next time.
• Faster with large libraries: product pictures are found without going through the whole library
  (1,000 pictures in a 100,000-item library: 10 seconds before, 0.05 now); opening the Library no longer copies
  the whole list through JSON (0.7 seconds to 3 milliseconds at 100,000 items); and during a sync each store's
  record is saved at most every 10 seconds, instead of after every product and file (a 5,000-product sync
  with nothing new rewrote a 1.4 MB file 5,000 times).
• The list of files Hoard has sealed never drops an entry (past 20,000 it could, in principle).
• Hoard for Unity 0.1.2: a .unitypackage whose headers claim impossible sizes is refused before anything
  is set aside for it. It's a separate release: see its own changelog.